This invention relates generally to a photo-typesetting machine and, more particularly, to a transport mechanism for delivering the photosensitive paper used in such machines.
Most printing machines employ some form of feed mechanism for either continuously or intermittently supplying a flexible sheet material that receives a print medium. The flexible sheet usually is maintained under tension so as to establish a predetermined alignment between successive lines of printed matter. Proper tensioning of the sheet material also is important in certain applications to insure stability of the sheet material during printing operations and thereby prevent blurring of the printed material. The latter factor is particularly significant for photo-typesetting machines in which images are optically produced on photosensitive sheet material.
A common passive type of tensioning system entails a pair of frictionally loaded tension rollers between which a flexible sheet is drawn by a suitable drive mechanism. Since the sheet material must be pulled through the tension rollers, the passive tensioning system has the deficiency of being effective only in those applications requiring a single direction of sheet material movement. For those applications requiring bi-directional sheet material movement, there have been employed and proposed semi-passive systems in which a tension roller is coupled to a one-way slip clutch and sympathetically driven by a main drive system. In a forward direction of sheet travel the tension roller is rotated by the frictional force of the moving sheet that overcomes the slip clutch. In the opposite direction of travel, the tension roller is driven by the main drive mechanism in the direction of sheet travel so as to maintain tension thereon. Although establishing tension for either direction of sheet material travel, the semi-passive tensioning system suffers the disadvantage of failing to provide dynamic tension in the absence of sheet material travel. This absence of tension at the completion of sheet travel after each phase of an intermittent feed system results in less than precise alignment of the sheet which can settle in a variety of positions within the backlash tolerance of the controlling elements.
Also known is an active tensioning system disclosed in U.S. Pat. No. 3,411,684. The active tensioning system utilizes an auxiliary motor that is connected to a tension roller by a slip coupling. During feed cycles, the frictional force generated by the moving sheet material overcomes the slip coupling to rotate the tension roller. Conversely, during static periods, the auxiliary motor drives the tension roller in a reverse direction to maintain tension on the stationary sheet. Although maintaining tension during both static and dynamic periods, the system disclosed in the previously noted U.S. patent exhibits the disadvantage of producing continuous rotation of the tension drive mechanism. In addition, the system fails to provide for bi-directional feed of the sheet material.
The object of this invention, therefore, is to provide an improved film transport mechanism for high speed photo-typesetting machines.